In recent years, considerable attention has been paid to improving the friction and wear characteristics of the sliding internal components of internal combustion engines, specifically piston rings and cylinder liners. For instance U.S. Pat. No. 5,549,086 to Ozawa, U.S. Pat. No. 5,618,590 to Naruse, et al., U.S. Pat. No. 5,743,536 to Komuro, U.S. Pat. No. 5,960,762 to Imaito, U.S. Pat. No. 6,139,984 to Onoda, et al., U.S. Pat. No. 6,315,840 to Onoda, et al., U.S. Pat. No. 6,553,957 to Ishikawa, et al., and U.S. Pat. No. 6,631,907 to Onoda, et al., touch on a wide variety of improvements in designs for ringed pistons and ringed piston cylinders and internal combustion engines. Much of this advancement has been directed to remedying the problems associated with the harsh operating conditions created by demands for higher engine output, demands created by the lightening of engine components and demands created by advancements in emission control.
One specific area of application that has not been addressed by these advancements is the operating conditions present in small displacement motors, like the small displacement glow ignition motors which are characteristically found in model cars, boats, and airplanes or small displacement motors in handheld devices like weed trimmers. These motors typically range in displacement from as small as 1 cc to 50 cc or more, with small displacement glow ignition motors generally being in the 1 cc to 10 cc range. Unlike the larger displacement engines, which typically utilize pistons and ring sets, these motors do not utilize piston rings. As a result, certain characteristics, which carry only minor significance for the larger displacement motors employing piston rings, become critical considerations in the design and operation of these small motors.
One such characteristic is the substantial power losses due to friction between the piston and the cylinder liner in a small displacement glow ignition motor. Typical pistons with rings have greater tolerances for fit and therefore friction, as the rings in the piston have a gap spacing within them allowing for an amount of travel in the ring that is not available in a ringless piston motor. The ring typically expands first in the mechanical sense, providing a tight fit around the piston at start up. The ring can then adjust when heated, providing a snug fit across all operating conditions. Moreover, in a typical ringed piston motor the friction surfaces presented by the ring are proportionately much smaller than in a ringless piston motor. The ring surrounding the piston makes the contact with the sleeve and, therefore, the losses due to friction are much smaller.
However, in a glow ignition motor because there is no piston ring that expands to seal the opening between the piston and the cylinder, the clearance between the piston and the cylinder liner must be extremely tight to insure proper compression of the air fuel mixture. In fact, the highest performance glow ignition motors will use an interference fit at or near top dead center. This extremely tight fit often necessitates pre-heating the motor to allow for a full rotation of the crank and, thereby, achieving optimal performance from the motor. Because of the tightness of the fit when the piston and engine are cold, measured power losses due to friction are substantial. The gradual increase in power can be readily seen as the engine temperature increases further increasing the clearance between piston and cylinder liner, as the liner expands faster, and decreasing the related frictional losses.
Another consideration is the catastrophic effect of wear on engine performance, especially in ringless piston engines. Because there is no expanding piston ring to allow for incremental wear on the cylinder surface, any wear at all in the ringless piston motor will result in less compression for a given operating temperature of the motor. The wear is further exacerbated by the extremely high operating speeds of these motors, which regularly exceed 40,000 revolutions per minute, as well as the aforementioned tight tolerances between piston and cylinder liner. Still another consideration is unit cost and manufacturing costs. Unlike large displacement motors typically found in passenger vehicles costing thousands of dollars or in aircraft worth millions of dollars, costs sensitivity in manufacturing these small displacement engines is significant in driving sales and marketability of the engines. Rare alloys and alloys difficult to manufacture with are cost prohibitive to use for this application.
One of the few recent developments in ringless pistons has been made by NASA as described in U.S. Pat. Nos. 5,769,046 and 6,116, 202 to Rivers, et al. These disclose an exotic carbon-carbon block and piston that can used to provide for a ringed or ringless engine. Carbon-carbon is both costly to manufacture and prohibitively difficult to work in, especially in the small dimensions required for small displacement engines, making it unsuitable for application in the small displacement motor market. Furthermore, the reference discloses that a coating can be applied, providing for instance a nickel coating on the piston skirt. There is little to suggest improved wearing low friction films or layers that might provide improved operation in a small displacement motor.
In fact, the suggested nickel coating processes is widely known in the small displacement motor industry. To confront the harsh operating environment in small displacement ringless piston motors and remain cost effective, it has become commonplace in the hobby craft industry to use a high silicon aluminum pistons coupled with brass cylinder liners possessing a hard chrome coating on the interior of the cylinder liner, commonly referred to as Aluminum, Brass, Chrome or ABC construction. The hard chrome in ABC construction is typically applied by electro-plating over the base of aluminum and brass. The hard chrome surface has improved wear characteristics over steel or cast iron, but even so has a short expected useful life. It is typical for these motors to show a measurable performance loss after only 1 gallon of fuel consumption, with many motors failing completely after 2 gallons. To date, no ringless piston or piston sleeve has been produced that takes advantage of the improved coating technologies and techniques available.
Therefore, a need exists for an improved ringless piston and/or piston sleeve that have fewer losses due to friction and increased durability over existing ringless pistons. More specifically, the need includes providing a lower coefficient of friction between the piston and the piston sleeve or motor with longer wearing surfaces to improve both engine performance and the life span through improved coating of the ringless piston and/or a piston sleeve.